Unmanned and robotic marine vehicles, such as remotely operated underwater vehicles (ROVs), autonomous underwater vehicles (AUVs), and autonomous surface vessels (ASVs), have become invaluable tools for marine exploration. They are commonly used for scientific, industrial, commercial, and recreational purposes. These vehicles generally use thrusters for propulsion.
A thruster is a device which provides a thrust force for propulsion or control of a vehicle. Generally, thrusters have an electric motor that is enclosed to seal it from water, a propeller rotatably connected to the motor through a mechanical seal or magnetic coupling, and a nozzle that protects the propeller and increases the thrust of the propeller. Thrusters often use oil compensation to allow operation at greater depths. U.S. Pat. No. 6,837,757 and U.S. Pat. No. 8,512,084 describe various embodiments of such devices.
Electric thrusters are powered through an electrical wire that connects the motor to a power source. In most cases, for variable thrust control, the power is supplied through an electronic speed controller. Such controllers are available for different types of motors such as brushed motors and brushless motors.
Synchronous electric motors are commonly used in a wide variety of applications. The thruster described here uses such a motor in the form of an external rotor brushless permanent magnet motor that is similar to those commonly used on model aircraft and computer disk drives. This motor design provides high torque at low rotational speeds without the need for a gearbox or reduction drive. It is simple and comprises a single moving part. The stator and motor windings are at the core of the motor and the permanent magnets are arranged in a radial pattern around the stator. Such motors typically have three phase magnetic windings that are commutated by an electronic speed controller. Sensorless feedback is used by the controller for proper timing of the commutation.
Common thruster designs are generally very large compared to an electric motor of equivalent power that is not sealed from the water. This is a driving factor in the design of many applications such as ROVs. The thrusters of an ROV (remotely operated underwater vehicle) often take up a significant portion of the total vehicle volume.
Electric thrusters are generally limited to depths of around 300-3,000 feet because they have air- or oil-filled cavities that are affected by the very high pressures experienced at depth. The vast majority of the ocean floor is at a much greater depth than this.
Electric thrusters, and in particular those that use mechanical seals, require maintenance as often as every 50 hours of operation to replace seals and add grease. Most ROVs have between three to six thrusters, making maintenance a laborious task. Additionally, for long endurance applications, such as long-endurance ASVs and AUVs, routine maintenance is not an option as the vehicles could be operated for months at a time.
Last, currently available thrusters are prohibitively expensive and consequently restricted from use in many applications such as hobby use, academic use, and small business commercial use. Much of the cost is driven by the complexity and number of parts used in typical designs.